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Feb 13, 2020, 06:12 PM
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No, the force IS there. So is the mass. I don't become weightless when I sit down. Or when I stand on a scale. Believe me, I wish I did.
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Feb 13, 2020, 06:25 PM
2 infect U it 1st has 2 find U
Miami Mike's Avatar
If an object is not being accelerated, that is, its velocity is not changing, it's because the net force acting upon it is zero.
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Feb 13, 2020, 06:27 PM
Registered User
Quote:
Originally Posted by Brandano
You can have a system of forces at equilibrium without actually having acceleration. The forces still exist, the masses still exist, and each action is matched by an equal and opposite reaction. This does not mean that there has to be acceleration, or indeed movement. Let's use one Newtonian law at a time, please.
Of course! That's why I wrote "if" ...
Quote:
Originally Posted by xlcrlee
... if the bodies are accelerated a force is implied in the defining formula (words have meaning).
Feb 13, 2020, 06:35 PM
Registered User
Yes, the NET force is zero. Because the opposing force balances the weight. Same way as lift on a plane balances its weight, and thrust balances its drag. And the plane can keep flying level at a constant speed.

But if you then add another object to the closed system that is starting static, and suddenly decides to accelerate downwards, then you need to find another force to justify it, because your accounting of forces doesn't work anymore.
Feb 13, 2020, 11:18 PM
Design is everything.
We need only to analyse the air mass between the nose and tail of the aircraft or beyond? Do we have to extend to infinity?
Feb 14, 2020, 04:28 AM
Registered User
insofar i understand it, the pressure differential which makes an aircraft stay aloft, are sortof like the charge on a battery, a imbalance locally. seen from afar, the forces cancel largely. the weight of an atmosphere and a aircraft, is the same wether flying or not. stationary, the wheels carry the aircraft. flying, the atmosphere "becomes" heavier and pushes a bit more on the surface. if one flies continuous loops, at a significant altitude, one can not distinguish a force or something from the surface. if one flies continuous loops, near the surface, the aircraft actually increases pressure between the surface and the aircraft, and decreases on the top of the loop. the net force is the same. the weight of atmosphere and aircraft the same. something comparable, is putting a small boat in surface water, which causes the global level to rise ever so slightly . this vanishes in other effects, if the water is some finite, large body compared to the boat. on a sidenote; it is very insightful for me, to have played with a old surf fin (a straight oldschool center fin) in water from the shore. one can visualise many vortex effects, and, strikingly, a significant thrust by mere twisting of the foil while moving. as i understand it, this is what causes a Darrieus rotor, to turn (in part at least).

edit: i wonder, if a airplane is flying in a airmass whereby one measures the pressure over a large area (weight of air and the aircraft in gravity), if one could detect where(!) the aircraft is, based on the net pressure over some area. i do think, one can measure the pressure wave ( - + - ) as the airplane passes (low) over some area. but i am not sure if one could measure, if the plane is over some specific area by measuring the pressure on that area.

edit 2: there may be something weird to this; something like, the heat of dissipation, and viscosity, somehow accounting for the reaction of the surface, of the aircraft flying. think of dropping a stone in syrup, "immediately" the weight is conducted to the container.

some other nice examples, are, a balloon heating up and rising. and, a submarine blowing water out and rising. nice thought experiment.
Last edited by m4rc3l; Feb 16, 2020 at 04:38 AM.
Feb 14, 2020, 06:51 AM
Registered User
You can put the boundaries of the system you analyze wherever you find it more practical, as long as you can budget for all forces and still manage to respect the 2nd law of thermodynamics. If you select your boundaries so that air exiting the system is accelerating downwards then the whole of the system should also be accelerating upwards or have an extra downwards force applied to it. In practice it's simpler to select your boundaries so that you account for all the air perturbation, including the air affected by the airplane while still in front of it. Air pressure after all travels at the speed of sound, so you can't expect air in front of the leading edge to be unaffected, unless you are in a ballistic regime.
Feb 14, 2020, 10:50 AM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by Designer2010
We need only to analyse the air mass between the nose and tail of the aircraft or beyond? Do we have to extend to infinity?
Obviously, it's 'to infinity and beyond'

.
Feb 14, 2020, 11:35 AM
Registered User
aeronaut999's Avatar
I hope that all you guys reviving this ancient topic have already sent in your comments to the FAA protesting their planned destruction of all model aviation in this nation.

https://www.rcgroups.com/model-aircr...-advocacy-943/
Feb 14, 2020, 06:59 PM
2 infect U it 1st has 2 find U
Miami Mike's Avatar
Like Pussycat said in Once Upon a Time in Hollywood, "Talk about a bring-down bummer, Dude."
Latest blog entry: A short message...
Feb 16, 2020, 03:11 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by Miami Mike
If an object is not being accelerated, that is, its velocity is not changing, it's because the net force acting upon it is zero.
Sorry, not buying that.

The net force between two forces can add to zero for a system and result in zero acceleration for the SYSTEM. But that doesn't mean the forces go away just because there's no acceleration. When I put my backside in the computer chair the force from acceleration due to gravity is balanced by the upward force from the chair and there is no movement. But the forces are still present. If the chair is pulled away by some nasty person the evidence of that existing force becomes apparent rather instantly.

Heck, if we want to expand on that and using your idea then we could say that there is no such thing as force since as a planet all forces that might cause motion are countered by other forces acting on them within the system that is our entire planet.

There are the laws of motion. And within that set of laws you're right. But that does not mean that static forces in balance are zero forces. It's just a different set of physics laws that governs static situations.
Feb 16, 2020, 03:56 PM
Registered User
As in this example, what we call a force is really only something we impute by observing change of motion, acceleration in some way:

240fps Slow Motion: Ball Suspended in Jet of Water (2 min 28 sec)


The ball is experiencing myriad collisions with water molecules that are bumping into the ball surface and like a person on a trampoline or tennis racket strings being distorted while smashing a being-distorted ball which we just normally don't see, don't think about or plain disregard these accelerations which are electromagnetically "elastically" dealt with by the the molecular structure of material as well the fluid version on the water stream and droplets.

When sitting on a chair it is only when our butt gets tired from the constant muscle and other action needed to molecule by molecule bounce against the chair seat .... but standing requires more obvious constant "vibrating" (muscles being pulsed!) effort which eventually tires one. This experiment where a cat balanced on a narrow plank falls into the water when she falls asleep shows this.

It is really the same, but more difficult, when this bird's center of mass is still, though clearly more effort than whe we sit or stand: it takes muscles AND there is definite acceleration going on!

Harrier hawk hovering (1 min 16 sec)
Last edited by xlcrlee; Feb 16, 2020 at 07:32 PM.
Feb 16, 2020, 05:20 PM
2 infect U it 1st has 2 find U
Miami Mike's Avatar
Quote:
Originally Posted by Miami Mike
If an object is not being accelerated, that is, its velocity is not changing, it's because the net force acting upon it is zero.
Quote:
Originally Posted by BMatthews
Sorry, not buying that.
Force is a vector, meaning that, like velocity and acceleration, it incorporates both magnitude and direction. When all of the forces acting upon an object are in equilibrium then the net force is zero and there's no acceleration. When the forces are not in equilibrium and a net force remains acting in a particular direction, acceleration occurs.
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Feb 16, 2020, 09:02 PM
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richard hanson's Avatar
The same interpretations and recitations keep poppingup
Most are true slices of the changes which occur to provide “lift”
For kids tho, a demonstration of the basic bit,which is a difference in pressure, is enough
No analogies, no comparisons needed
Just any demonstration of how changes in pressure can lift
If they delve more deeply, they will find that the basic fact, diferences in pressure, are always involved
Feb 16, 2020, 10:30 PM
Design is everything.
Quote:
Originally Posted by m4rc3l
insofar i ....

edit: i wonder, if a airplane is flying in a airmass whereby one measures the pressure over a large area (weight of air and the aircraft in gravity), if one could detect where(!) the aircraft is, based on the net pressure over some area. i do think, one can measure the pressure wave ( - + - ) as the airplane passes (low) over some area. but i am not sure if one could measure, if the plane is over some specific area by measuring the pressure on that area.

edit 2: there may be something weird to this; something like, the heat of dissipation, and viscosity, somehow accounting for the reaction of the surface, of the aircraft flying. think of dropping a stone in syrup, "immediately" the weight is conducted to the container.

some other nice examples, are, a balloon heating up and rising. and, a submarine blowing water out and rising. nice thought experiment.
There was a group of scientists who were able to estimate the mass of a jetliner flying straight by the pressure footprint on the ground . Could not find reference maybe jstor


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